An I/O interface is generally used for communication between two chips. In an actual communications system, a sending chip and a receiving chip cannot perform complete synchronous processing. A metastable state of the sending chip (the metastable state refers to an unstable state occurring when an I/O power supply is turned on or off) may cause a data writing error or a program running error to the receiving chip. As shown in FIG. 1, when a sending chip has only an I/O interface in an output direction, in order to prevent a metastable state of the sending chip from causing an error to a receiving chip, a weak pull-up resistor or a weak pull-down resistor (a resistance value is about a few tens of kilo-ohms, and whether a weak pull-up resistor or a weak pull-down resistor is selected is determined by a function of the receiving chip) is generally built in an I/O interface for an input direction in the receiving chip. A metastable state of a sending chip can be prevented from causing an error to a receiving chip, by building a weak pull-up resistor or a weak pull-down resistor in an I/O interface of the receiving chip.
When both a sending chip and a receiving chip have a bidirectional I/O interface, in order to prevent respective metastable states of the sending chip and the receiving chip from causing respective errors to the sending chip and the receiving chip, a built-in weak pull-up resistor or a built-in weak pull-down resistor should be disposed for the input directions of both the sending chip and the receiving chip. However, for an actual communications system, with a design of a sending chip used as an example, a weak pull-up resistor or a weak pull-down resistor needs to be built in a sending chip for an input direction in order to prevent a metastable state of a receiving chip from causing an error to the sending chip. However, this does mean that a weak pull-up resistor or a weak pull-down resistor is built in every chip for an input direction. During board design, because it is uncertain whether a weak pull-up resistor or a weak pull-down resistor is built in the receiving chip, it is still necessary to consider an error caused by a metastable state of the sending chip to the input direction of the receiving chip. In order to solve this problem, one pull-up resistor or pull-down resistor is generally connected to a board (whether a pull-up resistor or a pull-down resistor is connected is determined by a function of an input end of the receiving chip). As shown in FIG. 2, a weak pull-up resistor or a weak pull-down resistor is built in both a sending chip and a receiving chip, and a pull-up resistor or a pull-down resistor is also disposed on a board. Whether a built-in weak pull-up resistor or a built-in weak pull-down resistor is selected for the sending chip is determined by a function of the sending chip, and whether a pull-up resistor or a pull-down resistor is disposed on the board is determined by a function of the receiving chip.
According to an I/O interface-based signal output method in the prior art, even if a weak pull-up resistor or a weak pull-down resistor is disposed in both a sending chip and a receiving chip, and a pull-up resistor or a pull-down resistor is also integrated on a board, a problem of misoperations still exists. With a sending chip used as an example, all control signals for an I/O interface are generated by a core power supply domain. Even though a power supply of a receiving chip and an I/O power supply of a sending chip have been stable, when a core power supply of the sending chip is turned on or off, the I/O interface still collects some abnormal data from the core power supply domain and sends the abnormal data to the receiving chip, thereby causing some mis-operations on the receiving chip. These mis-operations affect a key control signal of the receiving chip, for example, a chip select (CS) signal/a chip select with low level (CSn) signal. Abnormal data, collected by the receiving chip when a core power supply of the receiving chip is turned on or off, may also cause the same problem to the sending chip.